Where is the Big Tree?

You do need Pro membership to merge profiles, which would link your ancestors to ancestors of other members (and presumably the big tree). If you have family members with potential matches, you can ask for help merging them here:
http://www.geni.com/discussions/99133

I should add that if this merge puts you over 100 family members, I think you will be unable to add more as a basic user

Thanks for the reply Josh, and the link. In the link, the Curators are offering to help merge profiles for non-pro members but are stating both managed profiles must be over 1000 members.

If I was to become a Pro member, can I merge with a manager of a Basic User? Or, is it only Pro members merging with each other?

Sandra - you are correct that you are NOT connected to the Big Tree. You do not have to be Pro to be a part of it. But - currently - Only Pro can approve a merge. Restricting helping folks merge to those with over 1000 profiles is a suggestion for their protection - since Basic only allowed 100 profiles in their family, and even Plus only allowed 1000.

If you became Pro, to merge with a Profile managed by a Basic User, either you would have to be in their Max Extended Family and/or their Family Group or become a co-manager of their Profile (the one you want to merge) or, if they make their Profile Public, be one of their Collaborators.

If you can find where you overlap a Public Profile on Geni**, folks can probably help you - either thru collaboration or becoming co-manager of your matching Profile or becoming (possibly temporarily) a "Family Member". You can possibly mention here or on the discussion mentioned above, once you find the match.

**To find overlap - possibly combine using Geni Search on unusual name or etc, and searching with Google while signed in to Geni (suggest -- include "site:geni.com" as well as the name)